Tubular electric heater



March 8, 1955 A. P. CHARBONNEAU TUBULAR ELECTRIC HEATER Filed Nov. 15, 1951 United States Patent TUBULAR ELECTRIC HEATER Allan P. Charbonneau, Wauwatosa, Wis., assignor to Cutler-Hammer, Inc., Milwaukee, Wis., a corporation of Delaware Application November 13, 1951, Serial No. 256,091

7 Claims. (Cl. 201-67) This invention relates to enclosed tubular type electric heaters and more particularly to an improved tubular electric heater having a temperature limiting cut-off device.

Tubular electric heaters are put to a wide variety of uses, one of which is the provision of quick controlled heat for the purpose of rapidly defrosting refrigerators. For this latter application the heater may be placed in abutting or spaced relationship with the outer face of the refrigerator evaporator where its generated heat is applied directly to the frosted surface or it may be positioned relative to the refrigerant so that the defrosting operation is accomplished by the circulation of the heated refrigerant. Because of the moisture involved and the temperature extremes attendant the use of said heaters, serious problems develop in designing a heater of the type which will withstand the conditions of service to which it is put. Another serious problem encountered is the provision of means insuring against overheating of the refrigerator evaporator should the defrost control device fail to function, in which case the refrigerator as well as its contents might be considerably damaged. Such protective means must be reliable, accurate, compact in size and economical to manufacture.

Accordingly, it is an object of this invention to provide an improved tubular type electric heater which is particularly suitable for use as a refrigerator defrost heater.

Another object of the present invention is the provision of a refrigerator electric defrost heater in which is included means for protecting against excessive refrigerator temperatures.

Another object of the invention is the provision of an improved refrigerator electric defrost heater having integrally included therewith means for interrupting the flow of current through said heater when the ambient temperature of a portion of said heater reaches a predetermined value.

More specifically, it is an object of this invention to provide an improved tubular type electric heater having included therein a temperature limiting cut-off device which is responsive to the ambient temperature of at least a portion of said heater rather than to the magnitude of the current flowing through said heater.

Another object of the invention is the provision of an improved tubular type electric heater having a builtin temperature limiting device which does not increase the diameter of said heater, is reliable and accurate in operation, and economical to manufacture.

Other objects and advantages of the invention will hereinafter appear.

The accompanying drawings illustrate an embodiment of the invention which will now be described.

Inthe drawings:

Figure l is a side elevational view of an improved electric defrost heater which is broken at the center to illustrate the similar construction of both ends thereof;

Fig. 2 is a view in longitudinal section of that end portion of the subject heater which includes the temperature limiting cutolf device, and shows said device in its normal or current carrying position; and

Fig. 3 is a partial longitudinal view of that portion of the electric heater shown in Fig. 2 and shows the temperature limiting device in its released or non-cur rent carrying position.

Referring to Fig. 1, it shows a tubular type electric heater comprising an outer metal sheath 10, end sealing plugs 20, and wire leads 25 which are covered with insulating material 26. End sealing plugs 20, which are made of a heat resisting and electrically insulating material such as neoprene or silicone rubber, are centrally apertured with an opening of just sufficient diameter to slip over the wire sheath material 26. Tight sealing engagement is obtained between the inner faces 20 of plugs 20 and the sheath material 26 by means of clamp members 27, and between the outer faces 20 of plugs 20 and the inner face 10 of metal sheath 10 by swaging the end portions 11 of the latter to reduce their diameter. (See Fig. 2.) Axial movement between the sheath material 26 and the sealing plug 20 is prevented by the clamp member 27 and the terminal lug 28 which connects the wire lead 25 to the terminal member 30 in a well known manner. Thus it will be seen that the heater is effectively protected against the entrance of moisture by a seal which is inherently flexible and has the advantage of lending itself well to the bending and twisting to which the heater is usually subjected when used as a refrigerator defrost heater.

Still referring to Fig. 2, it will be seen that the tubular metal sheath 10 encases a helical resistor 12 which is threadedly connected to terminal member 13 and insulated from said sheath by means of a compacted mass of insulating material 14.

Terminal member 13 is turned from brass or other good conducting material and has a portion 13 of slightly reduced diameter which is adapted to fit into the aperture of the electrical insulating washer 15 by means of which said terminal member is centrally positioned within sheath 10. Terminal member 13 also has a terminal portion 13 of further reduced diameter and a flange portion 13 which abuts the face of washer 15 to restrict the relative axial movement therebetween.

It will be noted that the fiber washer 15 abuts against a length of heat-resistant glass tubing 16, the outside diameter of which is just suflicient to permit its insertion into the metal sheath 10. The tubing 16, in cooperation with the fiber washer 15, the terminal members 13 and 30, and slotted fiber washer 17 which fits into the grooved portion 30 of terminal member 30, forms an enclosed chamher which is free of the refractory material 14. Within said chamber are the reduced heater terminal member portion 13*, a threaded sleeve 18, a helical spring 19 and the threaded portion 30 of terminal member 30. Slotted fiber washer 17 is used for convenience of assembly and restrains the terminal member 30 against the axial movement which would otherwise be induced by the tension of spring 19. Its outer face abuts against spacer sleeve 21 which may be made of material similar to that of sealing plug 20. Sleeve member 18 is preferably formed of brass and has a centrally positioned opening 18 for insertion therein of the reduced terminal portion 13 to which it is secured by means of a eutectic alloy 22 in a well known manner. Spring 19 is formed of a highly conducting material such as beryllium copper and is mounted under tension with one end threadedly connected to the threaded sleeve member 18 and the other end threadedly connected to the threaded portion 30 of terminal member 30. The dimensions of the terminals 13 and 30, sleeve member 18 and spring 19 are such that the temperature increase occasioned by the current flow through them is negligible compared to the heat generated by the resistor 12. Thus it will be seen that the temperature within the enclosed chamber, and consequently the temperature of the eutectic alloy 22, is substantially independent of the amount of current flowing through the heater. Rather, the temperature of the eutectic alloy Will correspond more closely to the mean ambient temperature of the heater, and dependent upon the thermal proportioning of the various parts the temperature of the eutectic alloy 22 will be substantially the same as that of the outer sheath 10. The purpose of this is to insure that the cut-off device does not respond to excess currents which may pass through the heater, but does function when the ambient temperature of the heater reaches the limit established by the particular circumstances of its application. The eutectic alloy selected for use in the cut-out device will generally have a temperature characteristic determined by the above limiting temperature.

There are several methods by which the sleeve 18 and the terminal member 13 can be made to disengage at different resistor temperatures. One such method 1s to use a eutectic alloy which has a different fusion temperature. Another method is to thermally insulate the resistor 12 from the alloy. One way of accomplishing this is to change the relative position of the resistor and the alloy. It has been found practical to straighten the resistor 12 for a short distance immediately adjacent the terminal member 13. This of course decreases the amount of heat generated near the eutectic alloy 22 and thus the sleeve 18 will not be disengaged from the terminal member 13 until the resistor temperature reaches a greater value.

The operation of the temperature limiting cut-out device contained in my improved tubular heater will now be explained. Referring to Fig. 2, it will be seen that the electric current carrying path extends from the heater terminal 30 through the current carrying helical spring 19, thence through the sleeve member 18 to the resistor terminal 13. The spring 19 is in its extended tensioned condition and is held against contraction by means of the eutectic alloy 22. However, should the ambient temperature of the heater exceed its desired limit, the eutectic alloy 22 will melt, permitting the sleeve 18 to slip free of the resistor terminal pin or portion 13'. As shown in Fig. 3, the spring 19 will then contract and interrupt the connection between sleeve 18 and terminal member 13, and consequently the electrical connection between said latter terminal and heater terminal member 30, thereby interrupting the flow of electric current through resistor 12 and preventing any further increase in the temperature of said heater. It will thus be seen that the operation of said cut-off device is simple, positive and reliable, and that electric tubular heaters into which said device is incorporated offer advantages heretofore not found in other similar heaters.

I claim:

1. An improved electric tubular heater comprising, in combination, a tubular outer sheath of substantially uniform dimension, a resistor positioned within said sheath, a first terminal member connected to said resistor, a second terminal member in substantial alinement with said first terminal member and having one end spaced therefrom, and a tensioned current carrying spring connected between said terminal members, said spring having a fusible connection with one of said terminal members, said connection being substantially thermally insulated from said resistor, whereby said spring is permitted to pull free of said latter terminal member when the ambient temperature of said heater exceeds a predetermined value and thereby interrupts the flow of current through said resistor.

2. In an electric tubular heater including an outer metal sheath of substantially uniform dimension and an electric heating element positioned concentrically therein, a temperature limiting cut-off device comprising, in axial alinement, a first terminal member, a second terminal member, and a tensioned current carrying helical spring connected between said terminal members, said spring having a eutectic alloy connection to one of said terminal members, said connection being responsive to ambient temperature, and said device being wholly enclosed by said sheath.

3. A tubular electric heater comprising, in combination, a tubular outer sheath, and positioned within said sheath: a resistor, a terminal member connected to said resistor and having one end thermally remote therefrom, a sleeve member having a eutectic alloy connection to said thermally remote end, a second terminal member spaced from said first terminal member, and a tensioned current carrying helical spring connected to said sleeve member and said second terminal member for interrupting the connection between said first terminal member and said sleeve member when the ambient temperature within said sheath reaches a value suflicient to melt said eutectic alloy connection.

4. A tubular electric heater comprising, in combination, a tubular outer sheath, and positioned within said sheath: an electric heating coil embedded in granular insulating material, a terminal member connected to said heating coil, a sleeve member having a eutectic alloy connection to said terminal member, a second terminal member spaced from said first terminal member, spring means connected between said sleeve member and said second terminal member, insulating means enclosing said spring means and providing for storage of power in the latter for interrupting said connection between said first terminal member and said sleeve member when the ambient temperature within said sheath reaches a value sufficient to melt said eutectic alloy.

5. For use with a tubular heater comprising a resistor having a terminal member, an open end tubular sheath member surrounding said resistor and at least a portion of its said terminal member, and a mass of refractory insulating material in said sheath wherein said resistor is embedded in spaced relation to said sheath member, the combination with a circuit connecting means, of a terminal lug clampingly engaging an end of said connecting means and connected to said terminal member, a perforated elastic plug formed of heat resistant material and located in part within an open end of said sheath member and having its inner periphery in engagement with said terminal lug, said plug being laterally compressed in situ by inward deformation of a portion of said sheath for annular sealing with the inner periphery of said sheath member and to prevent axial displacement therebetween, and clamping means on said plug to afford annular sealing between said plug and said connecting means to prevent axial displacement therebetween.

6. For use with a tubular heater comprising a resistor having a terminal member, circuit connecting means connected to said terminal member, an open end tubular sheath member surrounding said resistor and at least in part its said terminal member, and a mass of refractory insulating material in said sheath wherein said resistor is embedded in spaced relation to said sheath member, of a perforated elastic plug formed of heat resistant material located in part within an end of said sheath and laterally compressed in situ by peripheral inward deformation of the surrounding end portion of said sheath for annular sealing of said plug with the inner peripheral surface of said sheath member and the peripheral surface of said connection, and clamping means on said plug to afford tight engagement between the inner periphery of said plug and said circuit connecting means.

7. An improved electric tubular heater comprising, in combination, a tubular outer sheath of substantially uniform dimension, a resistor positioned within said sheath, a first terminal member connected to said resistor, a second terminal member in substantial alinement with said first terminal member and having one end spaced therefrom, terminal lugs connected to said terminal members, circuit connecting means clampingly engaged by said lugs, perforated elastic plugs formed of heat resistant material located in part within the respective open ends of said sheath member and having their inner peripheries in engagement with said terminal lugs, said plugs being laterally compressed in situ for annular sealing with the inner periphery of said sheath member and to prevent axial displacement therebetween, clamping means on said plugs to afford annular sealing between said plug and said connecting means to prevent axial displacement therebetween, and a tensioned current carrying spring positioned within said sheath, said spring having a fusible connection with one of said terminal members and being substantially thermally insulated from said resistor, whereby said spring is permitted to pull free of said latter terminal member when the ambient temperature of said heater exceeds a predetermined value and thereby interrupts the flow of current through said resistor.

References Cited in the file of this patent UNITED STATES PATENTS 1,203,125 Nuchols Oct. 31, 1916 1,359,400 Lightfoot Nov. 16, 1920 1,510,409 Maybaum Sept. 30, 1924 2,489,998 Charbonneau et a1 Nov. 29, 1949 FOREIGN PATENTS 2,538,808 Switzerland Jan. 23, 1951 931,560 France Nov. 3, 1947 

